Apparatus for implantation and extraction of osteal prostheses

ABSTRACT

Apparatus particularly suited for implantation and/or extraction of osteal prostheses, comprising an ultrasonic power generator, regulator means for regulating the ultrasonic power generator in accordance with a predetermined algorithm, at least one transducer connected to the output line for converting said electrical energy to a linear mechanical motion, an acoustic transformer for increasing or decreasing peak-to-peak motion of the transducer and means for connecting the acoustic transformer to a surgical tool or a prosthetic device. The system suitably comprises a pair of transducers, one operating at each of 20 KHz and 40 KHz frequencies. Novel adapters for the four major types of osteal prosthesis are also provided.

BACKGROUND OF THE INVENTION

This application relates to an apparatus for the implantation andextraction of osteal prostheses employing an ultrasonic energy sourceand designed for use by orthopedic surgeons.

A variety of techniques and apparatus for implanting and removingorthopedic prostheses have been developed since replacement parts forhip joints and like were first used well over half a century ago. Inparticular, total joint replacements have been carried out in a largenumber of patients for some time. While some improvements have been madeboth in the techniques for implanting the prosthetic devices and in theprostheses themselves, a need remains for improved surgical procedureswhich are less time consuming and minimize trauma to the patient.

Revision total hip arthroplasty for replacement of defective or damagedprostheses has in particular taken on increasing importance, as thenumber of patients requiring such revisions increases dramatically. Theprocedures currently employed often result in complications, many ofwhich are related to removal of the femoral prosthesis and theintramedullary cement mantle. The increased operating time required forrevisions is associated with increased blood loss, a higher infectionrate and increased postoperative morbidity. Femora perforation duringcement removal, with the possibility of intraoperative or postoperativefemoral fracture, is a well-known and particularly serious complication.

None of the previously available procedures for removal of the femoralprosthesis and cement mantle is entirely satisfactory. For example,while high speed cutting tools (such as the Midas Rex and Anspachpneumatic tools) have been found effective in cement removal, the usethereof may be hazardous due to the fact that the femoral cortex iseasily perforated. Image intensification may reduce the risk ofperforation somewhat, but is time-consuming and adds the additional riskof excessive radiation exposure. The alternative technique of controlledfemoral perforation requires wide soft tissue exposure and createsmultiple potential stress risers; as it has been argued that anycortical defect (including such deliberately induced perforations) couldincrease the chances of intraoperative or postoperative fracture, suchprocedure is clearly not without a potentially significant risk ofserious injury to the patient.

In an effort to provide improved procedures for the removal of thefemoral cement mantle, various new techniques have been developed. Forexample, a CO₂ laser has been employed for cement removal. In thiscontext, use of a laser has been found to have some significantdisadvantages. The instrumentation required is cumbersome; moreover,vaporization of the cement is slow, the fumes toxic and flammable, andthe potential for thermal necrosis of endosteal bone significant.Similarly, the lithiotriptor was explored as a potential tool forfracturing the cement mantle. It was determined, however, that the shockwave is difficult to focus, and thus microfractures of adjacent corticalbone occur frequently.

Systems using ultrasonic generators in conjunction with transducer orhorn elements have heretofore been developed for use in specific medicalapplications. A variety of ultrasonic tools are currently employedalmost routinely by practitioners in a number of fields, includingneurosurgery, ophthalmology and dentistry. As these devices are tailoredfor use in particular surgical applications, they are found to havelittle if any applicability outside the particular context for whichthey were designed.

Several systems have heretofore been developed for use in ophthalmiccataract removal, and phacoemulsification has become standard practicefor removal of cataracts. In addition, "CUSA" (cavitation ultrasonicaspirator) systems have gained some currency among surgeons involved inneurological tissue resections. Ultrasonic equipment is also in currentuse for scaling (removal) of calcified plaque from teeth and for tissueemulsification and homogenization. All of the above-described devicesare of limited applicability outside the particular context for whichthey were designed.

Ultrasonic devices have been employed for a variety of differentapplications outside the medical field as well. For example, ultrasonicapparatus has particular utility in the welding of plastics. Suchequipment would also clearly be unsuitable for use in the context ofsurgery, where delicate living tissue must be carefully manipulatedunder sterile conditions.

U.S. Pat. No. 4,248,232 (Engelbrecht et al.) suggests the use of anosteotome for removal of osteal prostheses. This patent, however, iscompletely silent with respect to the type of osteotome which would besuitable for use in such a context. Moreover, the patent fails toindicate any parameters whatsoever for the use of an osteotome inorthopedic surgery. Therefore, it is not surprising that there has beenno reported use of ultrasonic devices in the context of orthopedicsurgery to date in the medical literature. In fact, since the issuanceof U.S. Pat. No. 4,248,232 there has been a continued search foralternative techniques to solve the long-standing problems encounteredin the removal of damaged prostheses, as well as in the implantation ofnew prosthetic devices. Accordingly, there remains a need for apparatusthat would enable rapid and atraumatic removal of a prosthesis and/orthe cement mantle surrounding same, as well as the safe and efficientimplantation of prostheses.

SUMMARY OF THE INVENTION

The present invention has as its particular object the provision of anultrasonic apparatus which is specifically designed for use byorthopedic surgeons for implantation and removal of orthopedic devices.Pursuant to the present invention, it is possible for the practitionerin the field to achieve rapid and safe explantation of prostheses ofvarious types, including those with which a plastic cement (inparticular, polymethyl methacrylate or PMMA) has been used as anadhesive or filler. Implantation of prostheses is also greatlyfacilitated with the inventive apparatus. Moreover, the system of thepresent invention is designed so as to permit use of the device by anyorthopedic surgeon with minimal risk of injury to the patient after onlya brief familiarization with the equipment, which is clearly not thecase with the heretofore available equipment.

By virtue of the present invention, significant reductions in surgicalprocedure time are provided. There is also a reduction in trauma tosurrounding tissues and blood loss, as well as minimization of thepossibility of tissue necrosis. Further, the system operates in a mannersuch that the potential for weakening of the bone during the surgicalprocedure is reduced. As a consequence, the average hospital stay ofeach patient is shortened and the recovery rate increased, while theefficient use of the operating room facilities (in particular, OR floorspace) and personnel is maximized. The obvious attendant benefit of allthese advantages is a reduction in costs involved in the surgery aswell.

Further advantages of the present invention relate to ease and securityof operation. As the system has adequate reserve power to handle eventhe most difficult extractions, moreover, the amount of physical effortthe surgeon is required to expend is reduced. The level of expertiserequired of the surgeon to utilize the inventive apparatus issubstantially reduced, relative to that required for the complicated andoften dangerous procedures heretofore necessary, in particular forsuccessful explantations of prostheses which have been set using bonecement or when substantial bone ingrowth has occurred.

Pursuant to the present invention, direct coupling of the apparatus soas to transmit ultrasonic energy to a well fixed, cemented prosthesisallows for atraumatic removal of the prosthesis in a matter of seconds.Due to molecular friction and excitation created by transmission ofultrasonic energy, bone cement changes from a hardened state to a soft,plastic condition when contacted by ultrasonic tools. Tactile feedbackfacilitates distinction between the bone cement and cortical bone; inaddition, the cortical bone differs greatly in molecular structure,providing an additional innate protective effect. Removal of the bonecement from the intramedullary canal can be effected atraumaticallyusing the apparatus of the present invention, even in the case of porousimplants with substantial cancellous bone ingrowth.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates in schematic form one embodiment of the inventiveapparatus.

FIGS. 2A-2D illustrate four basic types of prior art hip femoralprostheses.

FIGS. 3A-3E illustrate novel adapters for use in extraction of hipfemoral prostheses in accordance with the present invention.

FIG. 4 illustrates the location of thermocouple sites on a cadavericfemur as employed in determinations of temperature during application ofultrasonic energy.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, there is provided in schematic form a descriptionof one preferred embodiment of the apparatus in accordance with thepresent invention. A hospital grade AC electrical connector 1 (either115V or 220V) is connected by means of low leakage AC power cable 2 to adual circuit breaker 3 (preferably, 15A) of either the thermal orelectromagnetic type. Cable 4 (generally, a low-leakage AC power cableas customarily employed for internal wiring) connects circuit breaker 3with an ultra isolation transformer 5 for leakage and AC line noiseisolation. The transformer is suitably adapted for 110-230V input (50-60Hz). The transformer will ideally have a 1 KW rating, with less than25.0 microamp leakage.

A regulator means 6 is provided, comprising in one embodiment a powervariable auto-transformer (i.e., Variac) provided to adjust the poweroutput limit. This device suitably has a nominal 10.0A rating. Inpreferred embodiments, regulator means 6 comprises control means (forexample, an appropriately programmed computer and associated hardware)for regulating the ultrasonic power generator in accordance with apredetermined algorithm. Regulator means 6 is connected to ultrasonicpower generator 7, which preferably has a 20 KHz center frequency. Whilegenerators are available over the range of 500-2000W, a 1 KW system ispresently preferred. This is particularly the case because somewell-embedded implants may require as much as 800W for the first 1 to 5seconds of removal. Higher power generators may be of advantage in someinstances, because they provide a larger bandwith of operation (± 10% ofthe center frequency) at reduced power output levels. This would permitless critical matching between the generator and other elements of thesystem, thereby making the system easier to use. A preferred ultrasonicpower system is available from Dukane, St. Charles, Ill. under thedesignation ULTRA 1000 AUTOTRAC; equivalent systems may of course alsobe employed, suitably with 115V input at about 13.0 ARMS maximum.

Control switch 8 is operative to turn on and off the high voltagegenerator output. A tone generator 9 is provided equipped with acommercially-available sound chip, amplifier and speaker. The tonegenerator 9 is operative to produce an indicator sound when the outputfrom the ultrasonic generator 7 is on. A 60 Hz (or 50 Hz) AC elapsedtime meter 10 is also provided for purposes of providing a maintenancehistory and for trouble shooting; a suitable time meter has anapproximately 10,000 hour capacity and 0.1 hour indicator.

The high voltage generator output is connected to a transducer orultrasound probe 11. The transducer 11 may be mounted in a handpiece(forming the transducer body) provided with a switch or operated by amedical grade footswitch. The transducer 11 may be sterilized using,e.g., ethylene trioxide; preferably, however, the transducer 11 isdesigned in a manner so as to be flash-steam autoclavable. Transducer 11is designed to operate at 20 KHz (2 KW, 1/2 lambda). This transducerfunctions as a solid state linear motor. The transducer 11 convertselectrical energy to a linear mechanical motion at 20 KHZ frequency withan approximately 0.0008 inch peak-to-peak excursion. The mechanicaloutput connection of transducer 11 is nominally a 1/2-20 set screw. At20 KHz with this design and composition, 1/2 lambda is nominally 5.36inches.

Acoustic transformer (sectional concentrator) or "horn" 12 increases ordecreases the peak-to-peak motion (stroke/gain) of transducer 11. Theacoustic transformer modifies not only the acoustic impedance, but alsothe quality factor Q. The quality factor defines a relationship betweenthe damping factor and stroke versus the frequency shift. A high Qreflects high acoustical efficiency and minimal self-heating. Both theshape and the composition of the acoustic transformer affect Q; astraight conical acoustic transformer made of annealed 303 stainlesssteel, for example, would have a low Q, whereas a "stepped" horn of7075-T6 aluminum would have a high Q. Generally, satisfactory horns areprepared from titanium or aluminum.

The length of the horn may vary greatly, depending on its design,composition and tuning frequency; typical lengths are on the order of4.5 to 6.0 inches. For an exponential horn made from 7075-T6 aluminum,the nominal length would be 5.36 inches (1/2 lambda); the nominalwavelength (lambda) is the product of the density of the material andthe speed of sound in the material.

Horn design is also determined by reference to the mode of operation.Thus, the shape of the horn will vary depending on whether operationsare carried out in exponential (low gain), catenoidal (medium gain) orstepped (high gain) mode. One particularly suitable acoustic transformeris a 6AL-4V titanium straight conical horn with a gain of approximately3, for a 2.4 mil peak-to-peak stroke maximum.

The acoustic transformer 12 is mechanically-acoustically connectedeither directly or by means of an extender 13 to an adapter 14. Extender13 serves the purpose of physically coupling adapter 14 to horn 12, aswell as to assist in matching the frequency of the transducer 11 plusthe horn 12 to the combination of the extender 13, adapter 14 and anassociated tool or implant. In addition, extender 13 may offertemperature isolation between the adapter 14 and horn 12, particularlywhen extender 13 comprises titanium. One suitable composition is 6AL4Vtitanium. The extender 13 may be on the order of 1 to 6 inches inlength.

Adapter 14 is used to connect the apparatus to an orthopedic implant 15(for implantation or extraction) or to individual tool bits 15 for usein various surgical applications. The primary purpose of the extender isto isolate the attached elements thermally while adjusting the nominalresonant frequency to 20 KHz ± 50 Hz.

As illustrated in FIG. 2, there are four basic types of hip femoralprosthesis in current use. Type 1 (FIG. 2A) comprises a stem 200 with afixed ball arrangement 201; the ball generally has a diameter of 22, 26,28 or 32 mm. A Type 2 prosthesis (FIG. 2B) has a so-called Morse taperstem 202; in addition, there is provided a horizontal extraction hole203. The Type 3 prosthesis (FIG. 2C) comprises a Morse taper 204 and avertical tapped hole 205. Finally, the Type 4 prosthesis (FIG. 2D) isprovided simply with a Morse taper 206.

In accordance with one aspect of the present invention, novel adaptersare provided which are expressly designed for use in conjunction witheach of the four basic types of prosthesis depicted in FIGS. 2A-2D. Theadapters are generally machined from stainless steel (e.g., 400 series),but may also be sintered, cast, etc. to the appropriate dimensions.Stainless steel is selected to lower the Q value at the implantinterface while providing adequate strength to keep the adapter/implantinterface from going into tension.

One suitable adapter for use in conjunction with a Type 1 prosthesis isillustrated in FIG. 3A. Adapter 300 comprises a base portion 301provided with a recess 302, the diameter of which is matched closely tothe diameter of the ball 201 of the corresponding Type 1 prosthesis.Suitably, the external circumference of base portion 301 is alsoprovided with at least one recess 303 to provide clearance on theadapter 300 for a lower portion of the prosthesis 200. A plurality ofcap screws 304 are provided in corresponding apertures 305 in the baseportion 301. These cap screws 304 cause a high compression interferencebetween the top of the ball 201 and the internal surface of adapter 300,as the faces of cap screws 304 contact the lower edge of ball 201 atsome point below the midpoint of the diameter from the top of ball 201.In a typical arrangement, contact occurs at approximately 3/4 of thediameter from the top of ball 201, whereby force is exerted at anapproximate 45° vector. The cap screws 305 are generally not necessarywhen the adapter 300 is used for implanting. Connection means 306 isprovided to permit facile assembly of adapter 300 with horn 12 and/orextender 13.

The adapter 310 illustrated in FIG. 3B is similarly designed for use inconjunction with a Type 2 implant. Base portion 311 is provided with arecess 312; portion 313 thereof is formed so as to provide a highcompression fit between adapter 310 and the portion of the implantsurrounding extraction hole 203. The upper portion 314 of recess 312 isdesigned to provide clearance for Morse taper 202. Cap screws 315 andcorresponding apertures 316 in the base portion 311 are provided toengage extraction hole 203; generally, cap screws 315 are provided withtapered ends. Adapter 310 is similarly provided with attachment means317.

FIG. 3C illustrates an adapter 320 of generally cylindrical shape foruse with a Type 3 prosthesis. One end 321 is provided with an aperture322 suitably machined to accommodate an end of a fastening means (suchas a screw), the other end of which may be securely attached to theaperture 205 in the Type 3 prosthesis. End 321 is appropriately angledto provide clearance for the Morse taper 204 when adapter 320 is inplace. An opposite end 323 is provided with an aperture 324, alsosuitably machined to permit coupling with the horn 12 and/or extender13. To facilitate attachment and removal of adapter 320, a pair ofindentations 325 (one of which is illustrated) are provided on thesurface of the generally cylindrical body; indentations 325 suitablypresent a flat surface for secure engagement of adapter 320 by a wrenchor other appropriate tool.

For use with a Type 4 prosthesis, adapter 330 (FIG. 3D) comprises a baseportion 331 provided with an offset throughhole 332 at an angle to matchthat of the Morse taper 206. Generally, the angle of the Morse taper ison the order of 135°; throughhole 332 is matched to this angle, andoffset sufficiently to match the taper in a line-line fit. A pluralityof apertures 333 are provided on the circumference of base section 331surrounding throughhole 332 for insertion of some type of fasteningmeans (e.g., set screws) to secure Morse taper 206 in place inthroughhole 332. In addition, the use of a suitable adhesive to fixMorse taper 206 in place may be advantageous. In a typical extractionprocedure, adapter 330 is brought into engagement with Morse taper 206and seated firmly, for example by tapping on surface e of adapter 330.After the setting of adhesive or other fixing agent, if any, employed tosecure engagement of Morse taper 206, fastening means introduced throughapertures 333 are brought into contact with Morse taper 206 so as tofurther strengthen the attachment of adapter 330 during the extractionprocedure. Once again, neither a fixing agent nor fastening means wouldbe necessary when the adapter is used for implantation of a prosthesis.

FIG. 3E illustrates a modified adapter 340 designed for use with a Type1 prosthesis, and particularly suitable for implantations. While lowerportion 341 is again provided with an aperture 342 designed toaccommodate the ball portion 201 of the Type 1 prosthesis, the adapter340 is further secured by means of a split nut 343, which obviatesdamage to the ball portion 201 of the prosthesis as might result fromthe use of a screw-type attachment. A bottom portion of split nut 343with a half cup matching the diameter of ball portion 201 is firstsnapped into place on the prosthesis; adapter 340 including a topportion that mirrors the split nut 343 is then applied over theprosthesis and the halves of split nut 343 brought into engagement. Toensure a good alignment of the halves of split nut 343, one or more pinswhich are inserted into match-drilled holes may be used. Once again,connector means 344 is provided for secure engagement with horn 12and/or extender 13.

Acoustic transformer 12, extender 13 and adapter 14 may suitably becoated with anodized (tiodized) processing, or using titanium nitride orboron nitride. Through the use of specific color coatings and/oridentification codes, as well as standardized fittings for allcomponents, the interchange of adapters and extenders for use with avariety of orthopedic implants and tool bits is rendered simple andessentially error-proof.

For implantation or removal of orthopedic prostheses, a directmechanical/acoustical connection is made via the appropriate adapter.While the adapters are of four basic types, variations are provided toaccommodate the differences encountered among the commercially availableprostheses with respect to implant angle, dimensions, ball size, holesize, and composition. To facilitate use by the surgeon, the system isdesigned so that it is necessary merely to identify the particular typeof prosthesis involved, whereupon the appropriate adapter (and, ifneeded, extender) may be selected from among those provided with theapparatus.

A separate circuit comprising power autotransformer 16, ultrasonicgenerator 17, tone generator 18, on/off switch 19, transducer 20 andacoustic transducer 21 is provided for purposes of operating anosteotome 22 or other tool bit. The elements 16-21 are of comparabledesign and construction as those of the first circuit described above,except that power auto-transformer 16 is designed to have a 6A maximumrating, and transducer 20 operates at 40 KHz (700W; 1/2 lambda=2.68inches). Osteotome or tool sit (tip) 22 is generally of stainless steelor titanium, and is used for modifying tissue or plastic; various shapesand sizes are provided for use as gouges, curettes, drills, etc.

A calibration circuit is provided, comprising an acoustic transformeranalyzer 23, a calibrated transducer 24, a calibrated acoustictransformer 25 (aluminum, precision calibrated to 20 KHz ± 20 Hz), asecond calibrated transducer 26 and a second calibrated acoustictransformer 27 (precision calibrated at 40 KHz ± 40 Hz). A preferredacoustic transformer analyzer is the Dukane Model 40A350 analyzer. Thecalibration circuit allows for testing and calibration of elements 11-15and/or 20-22 in whole or in part.

A torquing tray 28, generally in the form of a stainless steel tray andhold down clamps, is provided to hold the transducer firmly whileassembling the various components attached thereto (i.e., the acoustictransformer, adapter/extender and the tool element). An indicator lamp29, preferably with a voltage/current limiting resistor, advises theoperator when the apparatus is on.

Operation of the system is effected as follows. For an identifiedprosthesis, the appropriate combination of horn, extender (if necessary)and adapter is selected. Using torquing tray 28 and suitable wrenches,the combination of horn/extender/adapter is assembled and connected withthe transducer. The assembly is then connected to the prosthesis. A lowpower output (corresponding to a reduced stroke of, e.g., 20%) isselected for operation in a test mode and the range control is adjustedfor a minimum power reading on the power meter. If the reading isgreater than, e.g., 100W, adjustment is necessary before proceeding; ingeneral, adjustment of the frequency to 20025 ± 50 Hz (for example, bychange of extender) is sufficient to provide an acceptable power readingat low stroke.

Explantation may be effected by application of a suitable traction force(e.g., 10 pounds) on the transducer body housing and activation untilthe prosthesis has been removed (30 seconds maximum). Implantation intoa femur bed already prepared for an interference fit similarly calls forapplication of compression force, with activation until the prosthesisis fully seated. Automatic adjustment of operating parameters iseffected in a manner known per se by suitable circuitry provided withinthe ultrasonic generator equipment. The ultrasonic generator is designedto maintain a constant stroke. If the transducer encounters substantialclamping or holding forces, the ultrasonic generator provides additionalpower (up to its output limit) to keep the stroke constant. Similarly,an internal frequency control maintains a fixed phase relationshipbetween the driving voltage and the current, to adjust for shifts infrequency due to loading. Preferably, the ultrasonic generator isdesigned to provide a "softstart" feature, whereby initial stress on thetransducer and drive elements is reduced.

Well over one hundred different types of orthopedic implants arecurrently available for use in reconstruction of the hip, knee, elbow,shoulder, wrist, finger, toe, ankle, neck, etc. The inventive apparatusis designed to accommodate all of these various types of implants,simply by interchanging the adapters (and, where necessary, by using inaddition the appropriate extender). Further, the apparatus may be usedin conjunction with various types of T-bars (for use in removal of bonemarrow), broaches, files, gouges and curettes. In fact, virtually anytype of surgical tool (including those driven by hand, mallet,pneumatically, electrically or hydraulically) can be adapted for usewith the present invention. Most metallic, ceramic and plasticorthopedic implants (including those fixed with bone cement and poroustypes with substantial bone ingrowth) may be extracted or inserted intobone. Pins and rods used for bone fixation may be driven with theinventive apparatus, as may cutting or coring cannulae and curettes forbone, tissue or plastic modification. The inventive apparatus mayfurther be used for purposes of ultrasonic debriding. In addition, theapparatus may be used (with modified and new adapters) for dentalimplants, cosmetic surgery, Ob-Gyn and neurosurgical applications, etc.

In preferred embodiments, the workpiece may further comprise anirrigation/aspiration system with switch-over valves to an availablehospital air vacuum line and/or a biopsy trap. For purposes of fine boneand/or tissue resection, a smaller handpiece is provided.Pneumatic-guillotine cutters (e.g., occutomes, nucleotomes, biotomes,etc.), endoprobe accessories with camera capabilities and illuminators(ideally with disposable optical fiber cables) are additional elementsof the system available for particular uses. In addition to ultrasonicknives (with adjustable sharpness and drag), "hot knives" and/or uni- orbi-polar cautery electrosurgical knives may be provided.

A significant feature of a preferred embodiment of the present inventionis the use of either a touch screen CRT or flat panel display to inputcontrol and data functions. In addition, the display/control systemdisplays all systems parameters and performs the selection ofappropriate adapters and/or extenders for a particular use. The systemis ideally designed for infrared remote control from a sterile fieldand/or voice control of all system functions. In a further preferredembodiment, patient data and procedure parameter storage may beaccomplished using diskettes (e.g., "floppy" disks) compatible withoffice or hospital computer systems. A bar code reader system may beemployed. Magnetic or optical cards or disks may alternatively be usedfor input and/or storage of information. Repair or systems verificationtests may ideally be carried out by means of modem capability of thecontrol system.

In general, the algorithm for implantation or explantation of aparticular prosthesis is initially determined empirically, in view ofthe complex shape of each adapter/prosthesis combination and the widelyvarying acoustical performance of each of the range of prosthesescurrently available. Once the appropriate operating parameters have beendetermined, this information is incorporated into the system memory soas to permit ready access by the operator/surgeon. Thus, for example, inone type of embodiment of the inventive apparatus, the operator wouldselect the manufacturer and part number corresponding to the prosthesisin question. A display associated with the apparatus could provide anillustration of an actual-size implant for comparison purposes (e.g.,with a patient X-ray) and/or identify the appropriate horn, extender andadapter for use in conjunction with the given prosthesis. Alternatively,the necessary information concerning the appropriate combination ofelements for a given prosthesis could be recovered manually (e.g., usingcharts or tables).

To determine the appropriate configuration for any given prosthesisempirically, a first test adapter is machined to match a given implant.One of a series of standard calibrated extenders (for example, varyingin length over a range of 1 to 5 inches in 0.25" increments) is attachedto the adapter and to a low gain (exponential) test horn. This assemblyis then attached to analyzer means and the frequency noted. In the eventof a discrepancy between the measured frequency for the assembly and thetarget frequency of 20025 ± 25 Hz, the extender is changed and/or a newadapter is machined to an appropriate length; in practice, a differencein length of 0.001" corresponds to about 1.866 Hz. This process isrepeated until frequency parameters are met.

The assembly is then attached to a calibrated power generator andtransducer. At 100% stroke, the power should be less than 50 watts infree air and the stroke 1.5-4.5 mils (0.0015") peak-to-peak nominal. Ifthe stroke is not greater than 1.0 mils peak-to-peak at all points alongthe stem of the implant, the horn design is modified to increase thegain.

Following the above-described procedure, it is possible to determine theideal operating parameters for insertion or removal of any prosthesis,as well as for surgical operations using any type of tool attachment. Asan example, it has been determined that explantation of aCharnley-Mueller 32 mm fixed ball total hip prosthesis required 150 wattoutput at an approximately 2.0 mil peak-to-peak stroke for 7 seconds;this procedure resulted in a temperature increase in the adjacent boneof less than 3° C. Implantation of the same prosthesis requires about175 watts at the same stroke for about 3 seconds, and results in atemperature increase of about 5° C. or less. Removal of PMMA bone cementusing a curette (for example, a modified Zimmer 3670 curette) requires amaximum of 125 watts at an approximately 1.5 mil peak-to-peak stroke;approximately 2 mm of material is removed per second with a temperaturerise of about 7° C. or less.

In the development of the present invention, alternative technologiesfor explantation were also considered. One alternative was the use of anelectric current source with carbon-graphite electrodes attached acrossan exposed metal prosthesis, whereby the heat of the current flow acrossthe resistive metal prosthesis would eventually heat the entireprosthesis until the PMMA cement would soften and release theprosthesis. Another alternative was the use of focused ultrasound (5-100KHz) on the centerline of the prosthesis. The energy directed from atrough-shaped or phased array of piezoceramic elements would betransmitted into the tissue through a water bag technique as presentlyin use for lithotripsy. A large angle, short focal length transducershape would minimize localized tissue heating. Yet another alternativecontemplated was a modified water-cooled RF welder (3 KW, 450 KHz); aspecial ("welding/brazing") coil shaped to selectively heat the stemportion of a prosthesis until the PMMA interface softened. All of thesealternatives were rejected due to poor "fail-safe" modes of operationand/or possible undesirable clinical complications.

The invention will be better understood by reference to the followingexamples which are intended for purposes of illustration and are not tobe construed as in any way limiting the scope of the present invention,which is defined in the claims appended hereto.

EXAMPLE 1 Prosthesis Insertion and Removal

This experiment was designed to determine the effects of ultrasonictools and energy on endosteal bone during prosthesis and cement removal.Thermal necrosis is a common problem with various orthopaedicprocedures. Temperatures above 47° C. have been shown to createirreversible bone injury in vital microscopic studies, and alkalinephosphatase denatures at 56° C. Unfortunately, temperatures as high as100° C. have been recorded during in vitro drilling of cortical bone. Todemonstrate that the apparatus of the present invention permits a safeand rapid execution of the desired surgical procedures withoutsubstantial risk of tissue necrosis, heat generation was thoroughlyinvestigated according to the following procedure.

Ten cadaveric femurs, fresh frozen at -10° C. for 2-5 weeks, were thawedto 37° C. Thermocouple wires Type J (20 gauge) were inserted using a 2.0mm AO drill provided with a cortical depth caliper, and silicone sealantapplied to isolate the brazed tips from the water bath. Seventhermocouple sites 401-407 were employed, as illustrated in FIG. 4.Temperature and time were recorded on a 12 channel Graphtec Strip ChartRecorder (Model No. WRB 101/120), Tokyo, Japan. All channels werenormalized to 37° C.; calibrations were effected at room temperature,37° C., 46° C., and 55° C. using suitable standard laboratorythermometers before all trials, after the third trial, and at the end ofall trials.

During the prosthesis insertion and removal procedures, a 40 litergalvanized tank provided with a double screw holding clamp and filledwith 0.9% (normal) saline was used as a water bath. The bath was heatedand maintained at 37° C. using a heat flow pump (Model No. 73T availablefrom Polyscience Corp., Niles, Ill. Poly(methyl methacrylate) cement(Howmedica, Rutherford, N.J.) was prepared according to themanufacturer's instructions. A 32 mm fixed head Charnley prosthesis witha double-tapered stem was inserted into each femur; a 2-4 mm cementmantle was retained.

Temperature measurement began with prosthesis insertion. After curing ofthe bone cement and adjustment of the cement core temperature to 37° C.each prosthesis was removed using the apparatus of the presentinvention, operating at 200 Watt maximum power output and 100% stroke.The forces required for removal of the prosthesis were measured using astandard 100 pound spring-type hand-held tension gauge.

In all trials except a final one, the ultrasonic coupler/prosthesisinterface was tightly secured, in order to ensure efficient energytransfer and minimize the total energy requirements during removal. Inthe final trial, interface failure was created in an attempt to evaluatea hypothetical situation resulting in prolonged heating and ultrasonicenergy transmission. After prosthesis removal, a 2.5 × 2.5 cm area ofcement mantle surrounding the proximal thermocouple was removed usingultrasonic curettes and gouge, until all of the underlying bone wasexposed. Temperatures were recorded throughout the removal process.Three control femurs were prepared in a similar fashion; the prostheseswere inserted over a thin plastic sleeve and the prosthesis manuallyremoved after cement curing.

Temperature measurements at the thermocouple sites during cement curingand prosthesis removal are reported in Table 1. The mean T_(max) (°C.)recorded at all bone cement interface leads was 39.0 during ultrasonicremoval of the prosthesis, as compared to 39.9 during cement curing. Thecement core mean T_(max) (°C.) was 40.8 during prosthesis removal, ascompared to 47.9 during cement curing. The highest recorded cement coretemperature during ultrasonic removal of a prosthesis was 43.1° C., ascompared to a high of 66.3 recorded during cement curing. Temperatureelevations at the mid-cortical and periosteal leads were considerednegligible (<2° C.) throughout.

The mean pull-out force required for ultrasonic removal of a prosthesiswas 10 pounds; the range of forces was about 8-12 pounds. The mean timerequired for prosthesis removal was 9.7 seconds; the range of times wasabout 4.2-21.0 seconds.

During the trial in which ultrasonic coupler/prosthesis failure wasdeliberately induced, the mean bone cement/interface T_(max) was 46.5°C. Even under these unfavorable conditions, the mean pull-out force wasonly 22 pounds, and the time required for prosthesis removal 39.7seconds.

The 10 human cadaveric femurs were preserved in 10% buffered formalinfor 10 days prior to routine decalcification, preparation withhematoxylin and eosin, and sectioning for pathologic analysis by anexperienced bone pathologist. Attention was directed to the degree ofinduced osteonecrosis and thermal injury which occurred duringprosthesis removal (7 femurs), compared to the three controls.

Gross inspection of all specimens revealed no evidence of thermallyinduced eburnation of cortical bone. Curetted sites also revealed noevidence of cortical scarring.

Each specimen was analyzed by light microscopy for evidence of cellulardestruction in cortical lacunae and/or altered staining patternsindicative of thermally induced matrix damage. The depth of corticalbone damage was calibrated microscopically at all interface thermocouplelevels.

The mean depth of cortical damage at the cement cortical interface wasdetermined to be 6.1 cortical damage at the curette sites was 7.0micrometers (4.0-9.0 micrometers). Even in the trial where directcoupling to the prosthesis was deliberately suboptimal, the mean depthof cortical damage was only 14.2 micrometers (7-17 micrometers). Thecontrols had a mean depth of cortical damage of 2.0 micrometers (0.0-3.0micrometers).

The results confirm that direct coupling to a well-fixed, cementedprosthesis with an ultrasonic tool allows for rapid and atraumaticextraction of the prosthesis and rapid removal of the retained cementmantle without significant cortical damage. Maximum temperaturesgenerated were far below those which generate thermal necrosis. Noevidence was found of microfracture from ultrasonic energy transmission,and minimal cell injury was observed on a microscopic level. Theefficiency of this technique of prosthesis removal is demonstrated bythe short pull-out times and low pull-out forces required.

EXAMPLE 2 Ultrasonic Removal of Bone Cement

Eight freshly harvested canine long bone specimens (four humeri, fourfemoral) were prepared by exposing the intramedullary canals at theepiphysealmetaphyseal junction. The canals were broached in a standardfashion. Polymethyl methacrylate cement was prepared and digitallypacked into the intramedullary canals. After curing of the cement, thespecimens were allowed to harden at 10° C. for 72 hours. Ultrasonictools were then used to remove the cement completely from an interiorportion of the cement-filled area.

The bone specimens were sectioned using a diamond tip microtome into 1mm thick disks at points near both ends of the portion from which thebone cement had been removed, as well as from a control point remotefrom the cement-filled area. The disks were then prepared formicroradiographs and electron micrographs (20 KV, 10×). Themicroradiographs and electron micrographs of all disks demonstrated thepreservation of normal bony architecture.

From the foregoing description, one skilled in the art can readilyascertain the essential characteristics of the invention and, withoutdeparting from the spirit and scope thereof, can adapt the invention tovarious usages and conditions. For example, novel problems have arisenwith porous ingrowth prostheses, and removal thereof can be a highlymorbid procedure involving a proximal femoral osteotomy. Early clinicalapplications of the present invention have provided promising resultswith respect to the role of ultrasonic tools in porous ingrowth revisionarthroplasty. Changes in form and substitution of equivalents arecontemplated as circumstances may suggest or render expedient, andalthough specific terms have been employed herein, they are intended ina descriptive sense and not for purposes of limitation.

                  TABLE I                                                         ______________________________________                                        MEAN TEMPERATURE AT THERMOCOUPLE SITES                                        FOR HUMAN FEMORAL TRIALS                                                      THERMO-   CEMENT    ULTRASONIC  ULTRASONIC                                    COUPLE    CURING    PULL-OUT    CURRETTING                                    SITE      Tmax (°C.)                                                                       Tmax (°C.)                                                                         Tmax (°C.)                             ______________________________________                                        Proximal  40.3      38.9        38.7                                          Interface                                                                     Mid-Prosthesis                                                                          39.5      39.4        --                                            Interface                                                                     Distal    39.9      38.8        --                                            Interface                                                                     INTERFACE 39.9      39.0        --                                            AVERAGE                                                                       Mid-Cortex                                                                              38.1      37.4        --                                            Periosteal                                                                              37.4      37.6        --                                            Surface                                                                       Cement Core                                                                             47.9      40.0        --                                            ______________________________________                                    

What is claimed is:
 1. Apparatus comprising:an ultrasonic powergenerator including an output line for transmission of electrical energytherefrom; regulator means for regulating said ultrasonic powergenerator; a transducer connected to said output line for convertingsaid electrical energy to a linear mechanical motion at a firstpredetermined frequency; an acoustic transformer connected to saidtransducer for modulating peak-to-peak motion of said transducer;workpiece means for carrying out a desired manipulation, said workpiecemeans being selected from the group consisting of surgical tools andprosthetic devices; and coupling means for detachably connecting saidacoustic transformer to said workpiece means, said coupling meansincluding adapter means for isolating thermally the workpiece meansconnected thereto and for adjusting nominal resonant frequency in therange of 20 KHz ±50 Hz.
 2. Apparatus according to claim 1, wherein saidfirst predetermined frequency is about 20 KHz.
 3. Apparatus according toclaim 1, further comprising a handpiece forming a body for saidtransducer, said transducer being mounted therein.
 4. Apparatusaccording to claim 3, further comprising a manual switch for operatingsaid transducer.
 5. Apparatus according to claim 3, further comprising afootswitch for operating said transducer.
 6. Apparatus according toclaim 1, wherein said coupling means further comprises extender meansfor physically coupling said adapter means to said acoustic transformerand for matching the frequency of linear mechanical motion of thetransducer and the acoustic transformer to a resonant frequency for acombination of the extender means, the adapter means and the workpiecemeans.
 7. Apparatus according to claim 1, further comprising a separatecircuit including a second ultrasonic generator, a second transducerdesigned to operate at a second predetermined frequency, a secondacoustic transformer and second coupling means for connecting saidsecond acoustic transformer to a second workpiece means for carrying outa desired manipulation, said second workpiece means being selected fromthe group consisting of surgical tools and prosthetic devices. 8.Apparatus according to claim 7, wherein said second predeterminedfrequency is about 40 KHz.
 9. Apparatus according to claim 7, whereinsaid second workpiece means is an osteotome comprising stainless steelor titanium.
 10. Apparatus according to claim 1, further comprising acalibration circuit including an acoustic transformer analyzer connectedto said ultrasonic power generator, at least one calibrated transducercoupled to said acoustic transformer analyzer, and at least onecalibrated acoustic transformer coupled to said calibrated transducerand adapted for connection to a workpiece means to be calibrated. 11.Apparatus according to claim 1, wherein said regulator means furthercomprises control means for regulating said ultrasonic power generatorin accordance with a predetermined algorithm.
 12. Apparatus according toclaim 12, wherein said control means further comprises input means forinputting data and control functions.
 13. Apparatus according to claim11 wherein said control means further comprises a display.
 14. Apparatusaccording to claim 11, wherein said control means further comprisesselection means for identifying appropriate coupling means for apredetermined one of said workpiece means.
 15. Apparatus according toclaim 11, wherein said control means is an infrared remote control. 16.Apparatus according to claim 11, wherein said control means is a voiceoperated control.
 17. Apparatus according to claim 1, wherein saidultrasonic power generator maintains a constant stroke.
 18. Apparatusaccording to claim 1, wherein said ultrasonic power generator includesinternal frequency control means for maintaining a fixed phaserelationship between driving voltage and current, thereby adjusting forshifts in resonant frequency due to loading.